The use of far infrared viewers for surveillance and diagnostic work has become popular because of their ability to detect targets by their temperature difference in comparison with local backgrounds. This difference in many cases is impossible to conceal, and in diagnostic studies is often more pertenant than any visual changes that occur. Originally photographic techniques were preferred because of their high resolution, but current needs are running to realtime devices employing electronic circuitry. Excellent devices have been fabricated using individual diodes to generate the point or line elements that make up the final image. The resolution achievable depends on the size of the diodes and efforts to meet even modest requirements have resulted in very expensive devices. A more economical approach is to provide an imaging device with a continuous layered film or retina having a response property that can be measured by projecting a narrow beam of electrons onto its surface. These latter devices can be divided into two groups depending on whether the retina has a photoconductive or a thermoresistive (bolometer) type of response property.
In both types the retina is cooled to provide the necessary sensitivity, usually to the temperature of liquid nitrogen, and a shield at the same temperature blocks out essentially all radiation except that from the object space of interest. This results in a retina that is deeply recessed and requires a lens system with a narrow field of view. Even with such precautions there is still the vignetting effect of the cosine law refraction, which produces a radial shading of the temperature image at the retina. Superposed on this same image is the everpresent nonuniformity of the retina itself, which can obscure the small variations that take place between adjacent points on the retina. Similar restrictions apply to devices with both types of retinas, so that the resolution of the two are approximately equal.